Heat exchangers with cylindrical shells and helical tubes for heating fluids in the tubes are well known in the prior art. Typically the fluid flowing in the tubes is heated with a co-current flowing combustion gas provided by a burner located within the shell. These heat exchangers are typically high efficiency burners which are adapted for use in domestic applications. The exchangers are used continuously, and minor modifications which result in increased heat transfer efficiencies provide great cost savings.
There are two major ways to increase the efficiency of a heat transfer apparatus of the present type. The first way is to increase the conductive, convective and radiative heat transfer from the combustion gases to the water flowing through the tubes. This can be done by increasing the transfer time for heat transfer between the gases and the helical coils by diverting gas in the shell so that it remains in contact with the coils throughout the length of the shell. This can be accomplished by preventing the short circuiting of the gases leaving the burner. This increased heat transfer can also be accomplished by increasing the contact area between the coils and other heat conductive surfaces within the exchanger. Convective heat transfer can also be increased by increasing the Reynold's numbers of the combustion gases thus decreasing film thicknesses adjacent the heat transfer surfaces within the exchanger.
The combustion gases heat the entire exchanger and not only the coils carrying the water. Thus, the overall efficiency of the exchanger can also be improved by decreasing retransfer of heat to the exiting combustion gases from the heat conductive surfaces interior of the exchanger. This can be done by keeping the gases in contact with the coils throughout their path in the exchanger. The exiting gases can also be isolated from heat conductive surfaces thus decreasing both the convective and radiative heat transfer to the exiting gases.
Combustion gases produced by the burner of the heat exchanger are products of the reaction between air and a combustible gas. The reaction products which comprise the combustion gases are formed via a series of reactions between the reactants and intermediate products. The ultimate composition of the combustion gases relies very much upon the temperature at which these series of reactions takes place. Prior art heat exchangers have produced high concentrations of carbon monoxide and other products of incomplete combustion of the combustible gas and oxygen. These high concentrations of undesirable components result from a lowered reaction temperature caused by the cold fluid entering the heat exchanger adjacent the burner. Cold fluid draws heat from the reacting gases resulting in the incomplete products of combustion.
Thus, there exists a need in the prior art for an apparatus for transferring heat to fluids which optimizes heat transfer to the heated fluid, and preheats the entering fluid to reduce the concentrations of incomplete combustion products in the exiting combustion gases.